1. Field of the Invention
The present invention relates to an image heating apparatus, represented by a fixing apparatus provided in an electrophotographic image forming apparatus such as a printer or a copying apparatus.
2. Related Background Art
As the fixing apparatus provided in the electrophotographic image forming apparatus such as a printer or a copying apparatus, there is conventionally known a fixing apparatus in which a transfer material constituting a recording material and bearing an unfixed toner image is heated and pressed by conveying through a nip portion formed by a fixing roller constituting a fixing member provided therein with a heat source and a pressure roller constituting a pressing member and maintained in pressure contact with the fixing roller, whereby the unfixed image is fixed to the transfer material.
In an image forming apparatus of a relatively high speed, the above-mentioned fixing roller is usually composed of a metal core and an elastic rubber layer formed thereon. This is because the surface of the fixing roller is preferably flexible in order to improve contact with the unfixed toner image on the transfer material and to efficiently melt such unfixed toner image, as the transfer material employed in the image forming apparatus is often formed as a sheet with surface irregularities such as paper. Also the surface of the fixing roller is coated with fluorinated resin in order to facilitate releasing of the toner. Further, a web impregnated with a releasing agent such as silicone oil is maintained in contact with the fixing roller to form a thin oil layer thereon, thereby improving the property against toner offsetting.
The pressure roller is often provided with an elastic rubber layer thicker in comparison with that of the fixing roller, in order to form a predetermined nip in contact with the fixing roller. The surface of the pressure roller is also often coated with a fluorinated resin in order to facilitate releasing of the toner.
Also in order to resolve the offsetting drawback in such fixing apparatus, there is recently known an oilless fixing apparatus in which a potential difference is induced between the surface of the fixing roller and the pressure roller in such a direction as to press the unfixed toner image toward the paper thereby preventing the toner offsetting onto the fixing roller and dispensing with the oil coating member such as the above-mentioned web.
Such configuration allows compactization of the fixing apparatus, prevents defects such as oil leakage or oil blotting to the transfer material and reduces the toils of the user required for periodical replacement of a cleaning member.
On the other hand, in order to prevent wrapping of the transfer material around the fixing roller or the pressure roller, a separating claw is provided in contact with the surface thereof. In order to separate the transfer material of a thickness of about 100 xcexcm form the surface of the fixing roller or the pressure roller, the tip of the separating claw is formed as a small curved face of a radius R of curvature within a range of 0.07-0.1 mm in a cross section perpendicular to the axis of the fixing roller or the pressure roller. Such separating claw is maintained in contact with the fixing roller or the pressure roller under a predetermined pressure, in order to achieve satisfactory separation for the ordinary transfer material such as paper.
In the conventional fixing apparatus provided with the above-mentioned oil coating member, in order to separate the transfer material of a thickness of about 100 xcexcm from the surface of the fixing roller or the pressure roller, it is necessary to maintain a separating claw with a radius R of curvature of 0.07-0.1 mm in contact with the surface of the fixing roller or the pressure roller with a linear pressure of 0.196 N/mm or higher. However, in the above-mentioned oilless fixing apparatus, lacking the oil serving as lubricant between the separating claw and the surface of the fixing roller or the pressure roller, such surface tends to be abraded by the separating claw if the contact pressure thereof is high, whereby the service life of the fixing roller or the pressure roller is often shortened.
In order to avoid such drawback, there is empirically known a method of forming the tip of the separating claw into an extremely sharp form with a radius R of curvature not exceeding 10 xcexcm and reducing the contact pressure of the separating claw to the fixing roller or the pressure roller. However, if the radius R of curvature of the tip of the separating claw is simply reduced, such separating claw naturally pierces easily the fixing roller or the pressure roller provided with an elastic layer, and, in case of fixation of a recording material which is difficult to separate from the fixing roller or the pressure roller by the intrusion of the tip of the separating claw, such as a recording material bearing the image to the end thereof or an overhead projector transparency (OHT) sheet for color, surfacially coated with a resin layer, there may be applied an excessively large force on the tip of the separating claw and the fixing roller or the pressure roller may be fatally damaged. Also since the separating claw is rotatably supported, the tip of the separating claw may be inclined by the amount of play in the rotatable supporting shaft in case of defective separating operation, thereby resulting in so-called uneven contact phenomenon where one of the corner portions of the tip of the separating claw, in the axial direction of the fixing roller or the pressure roller, comes into contact with the surface thereof, thereby damaging such surface.
In the fixing apparatus (particularly oilless fixing apparatus) which separates the transfer material by contacting the separating claw with the fixing roller or the pressure roller provided with the elastic layer, there has not been found a condition capable of preventing fatal damage to the fixing roller or the pressure roller while attaining satisfactory service life of the fixing roller or the pressure roller under contact by the separating claw.
In consideration of the foregoing, an object of the present invention is to provide an image heating apparatus providing a satisfactorily long service life.
Another object of the present invention is to provide an image heating apparatus capable of providing satisfactory separating performance for the recording material, while preventing the damage on a rotating member.
Still another object of the present invention is to provide an image heating apparatus comprising:
a rotating member rotating in contact with a recording material; and
a separating claw for separating the recording material from the rotating member;
wherein the micro hardness A [xc2x0] of the surface of the rotating member and the radius R {mm} of curvature of the tip of the separating claw satisfy the following relationships:
Axe2x89xa7xe2x88x923250.0R2+90.5R+82.5 and 0.02xe2x89xa6Rxe2x89xa60.05.